Major oilseed rape disease pathogen shows widespread SDHI resistance

Scientists have found resistance to key SDHI fungicides in 95% of UK isolates of a major oilseed rape disease pathogen, raising concerns over the future control of Phoma stem canker.

The resistance was identified in Plenodomus biglobosus, one of two fungal pathogens responsible for the disease, and is the first confirmed case of SDHI resistance in the species anywhere in the world.

SDHIs, or succinate dehydrogenase inhibitors, are an important group of fungicides used in crop disease control.

Phoma stem canker can cause substantial yield losses in oilseed rape and is caused by P. biglobosus and the related pathogen Plenodomus lingam. While P. lingam has traditionally received more attention, P. biglobosus has become increasingly important in UK and European crops.

Researchers at Rothamsted Research screened populations of P. biglobosus collected from the UK and Poland as part of a study published in Pest Management Science.

They found 95% of UK isolates were resistant to the SDHI fungicides boscalid and fluxapyroxad, compared with just 6% of Polish isolates.

Some UK isolates also showed up to a 500-fold reduction in sensitivity to the fungicides under laboratory conditions.

The findings do not yet establish how much the resistance will affect disease control in commercial crops, and researchers say further work is needed to determine the impact under field conditions.

Dr Kevin King, who led the research at Rothamsted, said: “The scale of resistance we detected in UK populations was striking.”

He added: “This is the first confirmed report of SDHI resistance in P. biglobosus anywhere in the world. Further research is now needed to determine what impact this may have on phoma disease control under field conditions.”

The researchers linked the resistance to mutations in genes that encode the fungal succinate dehydrogenase enzyme, which is targeted by SDHI fungicides.

The study also found little evidence that these mutations reduced the pathogen’s fitness, suggesting resistant strains may persist in field populations.

The discovery adds to wider concerns over fungicide resistance in the pathogens responsible for Phoma stem canker.

Rothamsted has recently reported reduced sensitivity to azole fungicides in European populations of both P. lingam and P. biglobosus.

Researchers say the findings underline the importance of continued resistance monitoring and integrated disease management.

Professor Jon West, a co-author of the study, said: “This work highlights the importance of ongoing surveillance.”

He added: “SDHI-resistant P. lingam has recently been detected in Australia but not yet in Europe. Monitoring both phoma pathogens will provide early warning of emerging resistance threats and help protect the long-term effectiveness of fungicides.”